1. Field of the Invention
The present invention relates to method and apparatus for the real-time monitoring of a continuous weld using stress-wave emission techniques and, more particularly, to method and apparatus which monitors the harmonic content and the energy level of the stress waves emitted from a continuous weld to determine the quality of the weld.
2. Description of the Prior Art
Continuous welds, which can be defined as welds formed along particular paths or lines as opposed to spot welds, have in the past been evaluated after the weld was completed by, for example, scanning the weld area with systems using ultrasonic or X-ray techniques to detect flaws in the weld. The ability to evaluate a continuous weld using real-time, non-destructive methods has, therefore, always been of interest to industry.
A method for monitoring a spot-welding operation is disclosed in U.S. Pat. No. 3,726,130, issued to R. P. Hurlebaus on Apr. 10, 1973. There, ultrasonic shear wave-pulse signals are transmitted into the two pieces to be welded from a transducer positioned opposite the welding electrode while the welding operation is being performed. These signals are reflected from the area between the melting metal and the solid metal to provide real-time data for detecting the degree of penetration of a weld.
Another method for monitoring a spot-welding operation is disclosed in an article entitled "Forecasting Failures with Acoustic Emission," by R. E. Herzog published in Machine Design, June 14, 1973, at pages 132 to 137. There it is stated that one of the more successful uses of acoustic emissions is in inspecting welds as they are being made by detecting and correlating signals emitted during the liquid-to-solid phase transformation of a weld area to indicate good or bad welds. The Herzog article further specifies that complex stress waves occur in both the weld cycle and post-weld cooling period, but only emissions during the post-weld cooling period are used for finding defects, such as cracks, as they occur in the weld area, and that emissions during the weld cycle are ignored.
It is also known to detect and measure the stress waves emitted from a spot-weld area during n time intervals of the weld cycle, where n .gtoreq. 4, each interval corresponding to a different aspect occurring in the weld area during the weld cycle such as, for example, the initiation of heating, the solid-to-liquid phase transformation, the liquid-to-solid phase transformation, and post-weld cracking. The measurements obtained for the intervals are compared with predetermined acceptable ranges for measurements selected from corresponding ones of the intervals and the ratio between measurements of two or more of the intervals thereof to determine the quality and the extent of a spot weld.
The problem still remains of providing method and apparatus which will evaluate a continuous weld as the weld is being made.